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Geology of the Colorado Plateau

Geologists have subdivided the United States into physiographic provinces, each with its own distinct characteristics of landscape, mode of formation, and patterns of geology and geography. The Colorado Plateau physiographic province is one of the most unusual regions on the globe. This 130,000-square-mile region is a vast highland that lies above lower-elevation regions to the south and west, and abuts the higher country of the central and southern Rocky Mountains to the north and east.

Flat-lying rock beds, mostly of sedimentary origin, characterize the plateau. However, geological periods of folding, uplift, and erosion have given the plateau an irregular surface. Great blocks of the plateau have been uplifted along faults, forming the High Plateaus upon which Bryce Canyon and Zion are situated. Folds, anticlines, and synclines in the earth’s crust are largely responsible for the incomparable scenery in Capitol Reef, Arches, and Canyonlands national parks. Two erosional forms typify the Colorado Plateau. Mesas (meaning tables in Spanish) are remnant segments of the plateau isolated by erosion, generally flat on top, and bounded by steep cliffs. Buttes are smaller, cliff-edged prominences, with or without a flat top, that have been completely isolated and are detached from the main body of a mesa.

Sedimentary rocks are dominant in the national parks of Utah and the Colorado Plateau as a whole, with only a few scattered outcrops of igneous and metamorphic rocks. Most of the sedimentary strata in the region are flat-lying and rest in the same sequence in which they were laid down. Since the climate of the region is dry, little soil and vegetational cover have developed. Moreover, downcutting of the canyons has exposed to full view the full sequence of sedimentary rocks deposited on the Colorado Plateau. Like an open book, geologists are able to read this rock record, learning much about the climate and the environmental conditions that prevailed when the sediments were laid down. The presence of fossils here also increases our understanding of the development of life on the ever-changing earth.

More than 300 million years have elapsed since the rocks exposed in Utah’s national parks were deposited. The region included, at various times in the past, sea floors, floodplains, river deltas, swamps, coastal marshes and lagoons, sea coasts, and vast arid deserts. Yet nowhere in the geologic record is there an indication that the region earlier contained the great canyon systems it does today. Around the periphery of the plateau great mountains were uplifted and then eroded away, and other areas of the land in the plateau’s interior subsided into basins and low-lying areas that were periodically inundated by seas or freshwater lake systems.

Following regional uplift of the Colorado Plateau about 10–15 million years ago, erosion by the Colorado River and its tributaries stripped away much of the thousands of feet of lithified (consolidated) sediments, exhuming older strata that had been buried by younger sediments for millions of years. Today, weathering—whereby rocks are broken down into their constituent materials—and erosion—whereby these materials are carried away by wind and running water, are the principal earth-shaping processes affecting the land in the Colorado Plateau.

Formation of Arches

There are two categories of arches in Utah’s park lands: those with an opening below or behind the arch, and those without openings, such as the many large arches that are found high on massive sandstone walls in Zion National Park. Arches without openings are not considered in this discussion. Neither are natural bridges because, while arches are common in Utah’s national parks, natural bridges are not. They are formed as a direct result of a meandering stream cutting laterally into a narrow fin of rock, eventually cutting a hole through it. Such features are best seen in appropriately named Natural Bridges Natural Monument, in southeast Utah. In contrast, arches that develop openings below or behind them do so in the absence of streams.

Arches with openings are relatively rare in the world, but are quite common on the Colorado Plateau, and they are found in all of Utah’s national parks. There are two very common types of arches with openings in Utah’s national parks: 1) arches in a thin, vertical wall—or fin—of sandstone; they have a vertical opening entirely below the span; and 2) pothole arches, which have a horizontal opening. Both types of arches owe their existence to chemical and physical weathering by water.

In the first type the process of arch formation begins when groundwater seeps down through a porous sandstone layer, and it encounters an impermeable layer, such as shale. The groundwater then seeps outward along this interface of layers, eventually reaching the face of a cliff. There it dissolves the natural cement that binds sand grains into stone, and then water, wind and gravity remove the sand grains. Thus a small indentation begins to form on the cliff face at the horizontal interface between the two layers. This indentation grows horizontally along the interface and also vertically as rock immediately above the indentation is undercut and destabilized, and breaks away. Over time, more rockfall on a small or a grand scale causes the arch to grow. The indentation becomes larger, forming in the cliff what is called an alcove. If the body of rock behind the cliff is narrow enough, like the fins in Arches and Canyonlands national parks, the alcove will continue to penetrate the rock until it reaches the other side, and an opening forms. If two alcoves form on opposite sides of a fin, the process occurs much more rapidly.

The second type of arch, a pothole arch, is the most common type in Capitol Reef and Canyonlands national parks. A pothole arch begins to form when a pothole close to a cliff face collects water. Although much of the water trapped in a pothole evaporates, some seeps down into the porous sandstone below. When this groundwater reaches an impermeable layer, it seeps outward along the interface to the cliff face, and an alcove eventually forms there, in the same way as described above. The alcove and the pothole above continue to enlarge, and they eventually coalesce, forming a nearly horizontal opening called a pothole arch.